Add a general getting started guide.

The general getting started guide provides a single entry-point
into the installfest guides and properly pushes people into the
correct guide version for their Rails installation.

Updates the front-page link to head to the general getting started
guide instead of the Rails 3.2 guide.
Maybe sub/Mac Mavericks/OS X Mavericks/. It might also be useful to provide a link on how to find which version of os x you're using.
Taking a step back, i think it would be great if we were able to add expandable sections within the guide, most importantly with commonly encountered errors, but also maybe a 'more info' link that provides a bit more in-depth discussion about certain things.